There is increasing desire to find efficient and economical means for separation of high purity metal halides such as those of titanium, aluminum, gallium, silicon, etc. from domestic ores, and become less dependent on imported ores for these materials.
In the carbochlorination process for producing aluminum from clay, which is the largest domestic source of aluminum, the resulting aluminum chloride product normally contains from about 5 to about 10% titanium tetrachloride as a contaminant. Moreover, the use of distillation as a method of separating titanium tetrachloride from aluminum chloride has proven to be impossible because these compounds have similar vapor behavior at these low concentrations of titanium tetrachloride.
Documentation of the problems encountered in separating titanium tetrachloride from aluminum chloride is presented by A. Landsberg and H. C. Ko in an article entitled--On The Separation Of Titanium Tetrachloride From Aluminum Chloride, Met. Trans. B, vol. 17B, March, 1986, pp. 232-234.
One alternative to the distillation separation process is the selective condensation process, which uses a molten halide mixture as an absorbing bath; however, the procedure is encumbered by the disadvantage of requiring several subsequent distillation steps to recover the metal chlorides and regenerate the bath.
A second alternative to the distillation separation process is to absorb the metal chlorides in an aqueous solution and purify them by using standard hydrometallurgical methods; however, the disadvantages attendant to this process are that it occasions high energy consumption and gives rise to larger chlorine losses than the nonaqueous method. This is documented by S. J. Weiss in Recovery and Separation Of Products From The High-Temperature Chlorination of Fly Ash, Masters Thesis, Iowa State University, Ames, Iowa, 1980, pp. 32-58.
Further disadvantages in processes of separating aluminum chloride in the process of obtaining aluminum is that small amounts of ferric chloride are often times present, and this requires a large number of distillation steps to obtain acceptable aluminum purity. Also, in spite of the large number of distillation steps, some aluminum chloride is lost. Problems associated with this are treated by H. C. Ko, A. Landsberg, and J. L. Henry in The Vapor-Liquid Equilibria of the Aluminum Chloride-Ferric Chloride System, Met. Trans, B, vol. 14B, 1983, pp. 301-303.
In related processes of obtaining titanium, the titanium is often recovered from ilmenite or rutile ore by chlorination, and the resulting TiCl.sub.4 is purified by distillation, which has high energy requirements and cannot reject vanadium compounds. While various organic chemicals have been employed to prevent vanadium compounds from distilling along with the TiCl.sub.14, these organic compounds require several hours of contact under reflux to become effective and often leave undesirable compounds mixed with the TiCl.sub.4. See G. J. Dooley III, Titanium Production: Ilmenite vs. Rutile, J. Metals, Mar. 1975, pp. 8-16; also J. Barksdale, Titanium, The Ronald Press Co., 1949, 591 pp.
The invention's discovery is that, in certain supercritical fluids or solvents, TiCl.sub.4 is more soluble than aluminum chloride, and both of these compounds are more soluble than ferric chloride. Similarly, TiCl.sub.4, which has no permanent dipole is more soluble than VOCl.sub.3, which is polar.
Utilizing this discovery, the invention process employs supercritical fluids for recovering, separating and/or purifying metal halides.
The process of the invention may be used to purify titanium tetrachloride, which is needed to provide high-purity titanium metal for aerospace applications. The process of the invention may be used to purify aluminum chloride, which may provide aluminum metal for aerospace applications. Purification of materials important to the electronics industry, such as gallium and silicon may be also accomplished by this method.
The invention process can also be utilized to remove the above-mentioned metal halides from waste streams where the metal halides will cause harm to the environment if left combined with the wastes.